Will the equipment withstand irregular vibration?

IEC 60068-2-64, Test Fh provides a method for assessing equipment exposed to broadband random vibration. It belongs to the IEC 60068 environmental testing family and addresses the cumulative effects of dynamic loading on function and mechanical integrity. It serves a different purpose from a mechanical shock test, which applies a defined transient event.

Random does not mean uncontrolled

Equipment travelling in a vehicle experiences a changing mix of vibration rather than a perfectly repeating rhythm. In a random test, many frequency components act simultaneously, while instantaneous amplitude varies. The laboratory controls the specified spectrum; it does not simply shake the product arbitrarily.

A sinusoidal vibration test uses a single-frequency excitation at a time, either swept through a range or held at a selected frequency. Understanding how sine and random tests differ helps explain why they can reveal different weaknesses. Use the guidance on choosing vibration tests to relate the method to the expected service environment.

Read the spectrum, not just the headline g value

Power spectral density, or PSD, describes how vibration intensity is distributed over frequency. Acceleration PSD is commonly expressed in g²/Hz or (m/s²)²/Hz. “Power” here does not mean electrical power in watts.

Overall g RMS is the root-mean-square acceleration across the specified band, not the instantaneous peak acceleration. Integrating PSD over linear frequency gives mean-square acceleration; taking its square root gives RMS. For illustration, a flat spectrum of 0.01 g²/Hz from 20 to 120 Hz, with no contribution outside that band, gives √(0.01 × 100) = 1 g RMS. This is a teaching example, not a mandatory IEC test level.

Equal RMS values do not guarantee equal loading. Two profiles can concentrate vibration in different frequency bands and interact differently with the specimen's resonances. A meaningful specification therefore needs the full profile together with severity, duration and acceptance criteria.

The fixture is part of the test

The specimen is attached to a shaker through a mounting fixture that represents its intended installation. A control accelerometer measures the applied vibration, allowing the controller to adjust the shaker output towards the target PSD. Additional response measurements can help identify how the enclosure or internal assembly reacts.

For a PLC or sensor, orientation, fasteners, cable support and operating state can influence the outcome. A loose or unrealistic fixture can introduce an apparent product failure. Equipment requirements, including those addressed by IEC 61131-2 for PLC hardware, must be considered separately: citing Test Fh does not prescribe one universal profile for every product.

What a useful report should tell you

Exposure: Look for the frequency limits, PSD breakpoints, slopes and overall RMS. A statement that equipment “passed vibration testing” leaves too much unspecified to support a comparison.

Configuration: The report should identify the tested axes, duration per axis, mounting arrangement and measurement positions. It should also explain whether the unit was powered and which functions were monitored.

Evidence: Check the inspections before and after exposure, any interruptions observed during testing, mechanical damage and deviations from the planned profile. Acceptance needs predefined functional and structural limits. The IEC 60068-1 framework and the guide to selecting environmental tests for automation equipment provide useful context for planning.

Know what the result can support

A random vibration result does not establish resistance to rough-handling drops, hammer impacts or other mechanical shock conditions. Nor does it establish performance under temperature cycling or damp heat. Where service vibration includes strong periodic components or impacts, a purely random profile may be incomplete. Choose an exposure that represents the environment; increasing the level alone does not make a test more representative.

References and sources

IEC 60068-2-64:2008; IEC 60068-2-64:2008/AMD1:2019; IEC 60068-2-64:2026 RLV; Vibration Research — Understanding PSD; Vibration Research — Sine versus random testing